In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display panel or the like, a pattern is made by irradiating light to a semiconductor wafer or an exposure original plate for liquid crystal, but if a dust adheres to a photomask or a reticle used in this stage (herein collectively called “photomask”), the dust absorbs light or refracts it, causing deformation of a transferred pattern, roughened edges or black stains on a base, and leads to a problem of damaged dimensions, poor quality, deformed appearance and the like.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is still difficult to keep the photomask clean all the time. Therefore, the light irradiation is conducted only after a surface of the photomask is sheltered by a pellicle. Under such circumstances, foreign particles do not directly adhere to the surface of the photomask, but only onto the pellicle membrane, which is sufficiently remote from the surface of the photomask, and thus by setting a photo focus on a lithography pattern on the photomask, the foreign particles on the pellicle membrane fail to transfer their shadows on the photomask and thus no longer become a problem to the image transfer performance.
In general, a pellicle is built up of a pellicle frame, which is an endless frame bar usually made of aluminum, a stainless steel, or polyethylene, and a transparent pellicle membrane usually made of cellulose nitrate, cellulose acetate or a fluorine-containing polymer which transmit light well; this membrane is attached via dried solution or adhesive to one of the two annular faces (hereinafter referred to as “upper annular face”); furthermore, on the other one of the two annular faces of the frame (hereinafter referred to as “lower annular face”) is laid an agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like for attaching the pellicle frame to the photomask, and over this agglutinant layer is laid a releasable liner (separator) for protecting the agglutinant layer. FIGS. 1 and 2 show a construction of a general type pellicle.
In general, a pellicle membrane is a thin film made of a resin, so that in order to fix it on a pellicle frame in a slack-free manner, it is necessary to stretch the pellicle membrane to an appropriate extent as it is being fixed on the frame. Therefore, in general, in the case of a rectangular pellicle, which is generally used, the pellicle frame after being coupled with the pellicle membrane tends to curve inwardly to some extent.
This phenomenon is more conspicuous in the case of a large-sized pellicle, used for manufacturing printed circuit boards and liquid crystal display panels, for example, wherein the frame bars are relatively long, and also in the case of a small-sized pellicle, used for manufacturing semiconductor devices, wherein the frame is made of a material having low rigidity as is required for the reasons of limitations in material kind and size.
On the other hand, the photomask is required to provide as large an area as possible to be exposed to light for the reason of low cost. On account of this, the inward sagging of the frame bars is a problem as it decreases the area of the photomask available for light exposure. Also, in the case of a pellicle wherein the contour of the pellicle frame as seen from above is too deformed from rectangle, that is, if the external sides of the frame are not straight enough, it gives rise to a problem of lowered precision in mounting the pellicle in the predetermined site on the photomask.
It is not only the inward direction of the pellicle frame, in which the tension of the pellicle membrane is oriented, that the sagging of the pellicle frame bars becomes a problem; but also in the direction of the thickness (or height) of the pellicle frame, in which direction the pellicle frame bars can sag during pellicle manufacturing or during a handling step prior to pasting of the pellicle membrane, if the thickness (height) of the frame bars is small in comparison to the lengths of the frame bars. Especially in the case of an extra large pellicle frame wherein the frame bars exceed 1000 mm in length, the frame bars would sag downwardly conspicuously in the middle when the pellicle frame is held horizontally by being supported at the four corners for transportation, and this would considerably thwart a smooth handling and in some cases would result in a permanent deformation of the frame.
In the past, Publication-in-IP 1 proposed a means for solving the sagging problem of the pellicle frame, according to which one parallel pair of the frame bars are formed in a manner such that the middle part of respective bar is convexed outwardly in a shape of an arc, which part is adjoined on either end by a concave part in a shape of an arc, which in turn is adjoined by straight part. However, although this means is useful in preventing an inward sagging of the pellicle frame caused by the pellicle membrane tension, it is not useful in solving the problem of sagging in the thickness-wise direction of the pellicle frame, for it does not increase the rigidity of the pellicle frame.
An answer to solving this problem is increasing the rigidity of the pellicle frame, and there are two ways of doing this, namely, increasing the cross sectional area of the pellicle frame bar and selecting for the pellicle frame a material which is high in elasticity modulus.
The method of enlarging the cross sectional area of the pellicle frame bar, that is, the dimensional expansion, is not appropriate in that an inward expansion would narrow the exposure area of the photomask and in that an outward expansion would make it difficult to secure a sufficient amount of clearance for the fixation of the photomask and for handling at the time of transportation. Furthermore, in the thickness-wise (height-wise) direction, there is a restriction imposed by the light exposure apparatus that the height of the frame bar must be from 3 to 8 mm or so. Therefore, there is virtually little freedom for the dimensional designing and thus it is difficult to resort to a dimensional change to increase the rigidity.
On the other hand, as the way for increasing the rigidity of the pellicle frame by using a material having a higher elasticity modulus, there are proposed alternatives such as steel, stainless steel, titanium alloy, and aluminum alloy reinforced with a built-in substance having a higher rigidity, to replace the conventional aluminum alloy (Publication-in-IP 2). However, iron-based alloys such as steel and stainless steel are too weighty, and titanium alloy is very difficult to grind into shapes so that they are scarcely used in practice. As an alternative to these, carbon fiber reinforced plastic (CFRP), which is increasingly used in the aircraft industry and is known as a high rigidity material, is expected to extensively improve the rigidity of the pellicle frame, more than the above-mentioned materials, and that without being too weighty and too difficult to grind in shapes. However, it has been greatly feared that by using carbon fiber the possibility of creating foreign particles would be high, and hence this has not been adopted.